Imaging systems comprising a leuco dye, nitrate ion, and diazonium salts in a binder were disclosed in U.S. patent application Ser. No. 101,196 filed Dec. 7, 1979, abandoned. That system provides a light-sensitive, thermally developable, negative acting system. That is, the optical density in the final image is more dense in areas where light struck than in areas which are not light struck. The exact phenomenon by which that light-sensitive element functions is not completely understood.
Imaging systems comprising a leuco dye and nitrate ion in a binder are disclosed in U.S. patent application Ser. No. 101,197, filed Dec. 7, 1979. That system provides a light-insensitive, thermally developable system which provides optical density where heated.
Many other processes and compositions use leuco dyes to provide optical densities in the imaged article. For example, U.S. Pat. No. 4,017,313 uses a combination of a photosensitive leuco dye, a photo-sensitizer for the dye, an aromatic aldehyde and a secondary or tertiary amine. Other photosensitive systems using leuco dyes are included in U.S. Pat. Nos. 3,390,997, 2,884,326, and 2,772,284. The mechanism of these last two patents are disclosed in "Aromatic Aldehyde-Leuco Dye Photooxidation," H. D. Hartzler, Pure and Applied Chemistry, Vol. 49, pp. 353-356, Pergamon Press, 1977, Great Britain.
Light-Sensitive Systems, J. Kosar, 1965, John Wiley and Sons, Chapter 8, page 369, describes print-out photosensitive systems comprising a binder, leuco dye, organic halogen compound and photosensitizing dye. Because these are printout systems, there is no thermal amplification.
A great many photosensitive materials have been used in different imaging processes utilizing various photoinitiated phenomena such as photohardening of polymerizable materials (e.g., negative acting printing plates), photosolubilizing materials (e.g., positive acting printing plates), light initiated diazonium salt coupling reactions (e.g., diazonium microfilm), etc. A recently discovered class of iodonium photoinitiators for both cationic and epoxy polymerization (e.g., U.S. Pat. Nos. 4,026,705, and 3,981,897, and U.S. patent applications Ser. Nos. 467,899 filed May 8, 1974 and 40,645 filed May 21, 1979 in the name of George H. Smith) has also been proposed as equivalent to other photoinitiators in certain ethylenically unsaturated printing plate compositions (e.g., U.S. Pat. No. 3,741,769).